1. Field of the Invention
The present invention relates to an alumina-film-polishing composition and a chemical mechanical polishing method using the same. The composition and the method of the present invention are employable in the planarization process to form an insulating layer of a magnetic head device for use in a magnetic recording apparatus.
2. Description of the Related Art
A magnetic head device for use in a magnetic recording apparatus is manufactured by forming a plurality of elements, each including a magnetic reading section and a magnetic writing section, on an alumina-titanium carbide ceramic substrate (hereinafter referred to as “AlTiC wafer”), and subsequently cutting the substrate into the individual elements. The basic structure of the elements is formed through various wafer processes performed on the AlTiC wafer, as is the case with a semiconductor device using a silicon wafer. One of such processes is chemical mechanical polishing (hereinafter referred to as “CMP”). In the CMP process, an object to be polished is brought into contact with a polishing pad with a down force while the object is rotated relative to the polishing pad, so that an irregular surface of the object is planarized. A polishing composition (polishing slurry) containing an abrasive grain and a polishing accelerator is used in the CMP process.
In a magnetic head device, unlike a semiconductor device, alumina (Al2O3) is used for most insulating layers. For this reason, polishing slurries for use in a CMP process in manufacturing a semiconductor device, which are typically intended for polishing silicon, cannot be used for such insulating layers. A polishing slurry most commonly used in the CMP process in manufacturing a magnetic head device has a composition containing an alumina abrasive grain with a cation or anion added thereto, the cation or anion having a capability of dissolving alumina, which is the object to be polished, to some extent.
In forming an element having a multilayer structure, an insulating film is formed between a lower layer and an upper layer. One of major purposes of the CMP process is to eliminate steps (differences in level) on the surface of the insulating film resulting from irregularities of the lower layer, and to thereby prevent a deviation of the exposure focus when patterning the upper layer. In the CMP process, a difference in removal rate between higher (projecting) areas and lower (recessed) areas of the surface is utilized for reducing the step heights. The step heights are more efficiently reduced as the difference in removal rate is greater. As the step heights are reduced to some extent, however, the difference in polishing down force between the higher areas and lower areas becomes small, so that the lower areas, which need not undergo removal, highly tend to undergo removal together with the higher areas. To completely planarize the irregularities, a stock removal nearly twice the initial step height is therefore required. Typically, as the stock removal increases, the “stock removal variation” increases accordingly. The “stock removal variation” means variations in stock removal in a wafer surface polished, and is expressed as a difference between the maximum stock removal and the minimum stock removal in the wafer surface. This term is used in the same sense throughout the following description. Polishing a surface can cause a certain stock removal variation, due to the act of polishing, even when a wafer with a completely flat surface is polished, and can result in higher (projecting) areas and lower (recessed) areas in the surface. It is therefore extremely important to reduce the stock removal variation in manufacturing products. Unfortunately, however, increasing the stock removal in order to eliminate the initial step height inevitably increases the stock removal variation.
One approach to solve this problem is to form a polishing stopper layer, made of a material resistant to removal by polishing, on the lower (recessed) areas before polishing is performed. For example, JP 2004-349426A discloses a technique of using a polishing slurry that can provide a high removal selectivity ratio between the polishing stopper layer and a layer to be polished. This technique is considerably effective, but disadvantageously involves an increased number of process steps. In addition, since this technique is intended for a process to polish a silicon dioxide film, it is questionable whether this technique is applicable to a process to polish an alumina film. This is because the polishing slurry disclosed in JP 2004-349426A, which has a satisfactory removal selectivity ratio for a silicon dioxide film, will not necessarily exhibit a practical removal rate for an alumina film.
To solve the above-mentioned problem, furthermore, an attempt has been made to change the polishing process itself. A typical polishing process is performed on an elastic polishing pad. Consequently, a deformation of the polishing pad causes transfer of some polishing down force to lower (recessed) areas, as well as higher (projecting) areas, of the surface being polished. To cope with this, various techniques have been proposed, including a technique of employing a polishing pad harder than a conventional one, and a technique of employing a polishing process with a lower polishing down force and an increased linear velocity in order to minimize the deformation of the pad. While these techniques may be effective to some extent, they cannot be a fundamental solution because each of them relies on the same principle of polishing. Rather, these techniques may lead to an increase in stock removal variation in the wafer surface.
JP 2006-516067A discloses the use of a polishing composition that contains a water-soluble polymer compound for the purpose of reducing the occurrence of scratches during the CMP process. However, this composition is also intended to be used for polishing a semiconductor substrate, and therefore cannot be directly applied to polishing of an alumina film. In addition, in this publication no attention is drawn to reducing the total stock removal required for complete planarization.